This invention relates to an apparatus for and method of presetting plate cylinders to eliminate registering errors in an offset printing press before a printing operation.
An offset printing press for multicolor printing ordinarily has four printing units each provided with a plate cylinder, a blanket cylinder, etc. A printing plate is mounted on the outer peripheral surface of the plate cylinder of each printing unit. It is impossible to mount all printing plates on their plate cylinders with exactly the same registering phase relationships. That is, there are always some slight phase deviations in the rotational, lateral and oblique directions among the plate cylinders. The phase deviation in the oblique direction means that a printing plate is mounted obliquely on its plate cylinder in a twisted state.
In order to match the phases of all plate cylinders heretofore, a certain number of the plate cylinders have been slightly adjusted for registering while printing test (proofing) is carried out many times. This procedure takes much time and causes much waste of paper.
To avoid this, there have been proposed a variety of apparatuses for presetting multiple-color plate cylinders for registering.
For example, there is a presetting apparatus for registering which has a computer. The computer once memorizes the magnitude of register adjustment at the time when each plate cylinder is set in a correct position according to the kind of printing paper. When the same kind of printing paper is thereafter used, each plate cylinder is preset for registering on the basis of data previously memorized in the computer.
In addition to the above conventional apparatus, there is another conventional presetting apparatus which has a plurality of sensors for detecting register marks formed on a printing plate mounted on each plate cylinder. Each sensor detects the phase deviations of the plate cylinders to carry out a registering operation.
In the former conventional presetting apparatus, however, whenever a new printing plate is mounted on each plate cylinder, the mounting operation of the printing plate newly causes phase deviations. The phase deviations due to one mounting operation are different from those of other mounting operations. Accordingly, the data for a presetting operation previously memorized in the computer are not necessarily applicable to a new printing operation even if the same kind of printing paper is used because of a difference in the phase deviations among a plurality of the mounting operations. This results in carrying out the printing test (proofing) many times.
The latter conventional apparatus also requires repeated proofs because of the following reasons.
As a rule, when a web or a continuous printing paper is used in an offset printing press, the web passes between a pair of blanket cylinders in each printing unit so that ink on the blanket cylinders is transferred to the surface of the web. At this time, there often occurs a delamination which means that the web is pulled by either the upper or lower blanket cylinder in the rotational direction thereof because the web is caused to adhere to the surface of the blanket cylinder by the viscosity of the ink. As a result, if the delamination occurs, there will be a change of the length of the web extending between the two adjoining printing units. This causes a register error in the rotational direction of the plate cylinder, which register error changes in accordance with printing conditions such as the tension, the width, the weight per unit length of the web and the area of patterns on a printing plate.
In the latter conventional presetting apparatus, the presetting operation has not been carried out in consideration of the delamination and proofing has been carried out before a normal printing operation in order to eliminate the error caused by the delamination.